Hidden Sniper Rifle

The first Sniper Rifle/weapon stash is located directly after the Hunter battle. When you walk into the clearing where the obvious highway used to be look to your right and there should be two boxes. Jump on them. To your right should be the stash. You should know it when you see it because several dead Jackals, Grunts and Humans will be lying there (signs of struggle) as well as pistols and rifles. Also, this is the area where the Pelican picks up Sergeant Johnson (and drops off new Marines). The end of the next alley, nicknamed Sniper Alley, has a hidden Sniper Rifle.

First, start the mission until you reach "Sniper Alley" (after Sgt. Johnson is picked up and you are reinforced).

Second, kill all Drones and Sniper Jackals in the area, so you'll have more time if you can't find it.

Third, go the far side of the alley, which is ruined and in flames. You should see a dead Marine on the right side with a Sniper Rifle and ammo pouches. Pick up the sniper, (which will have low ammo), and after collecting all ammo pouches from the Marine, you should now have full ammo for the Sniper Rifle.

Fourth, you may not be able to use the Sniper much for the remainder of the mission, since there will be Warthogs on the next section of the mission, but you may be able to take the sniper to the next mission, Metropolis.

If you get on the roofs in Sniper Alley, look for two buildings connected by a bridge. In one building there is a crate, and in the others balcony there is a Sniper Rifle, and inside is a bunch of ammo and posters. This is also where the IWHBYD Skull spawns if you are trying to spawn it.

Also, where the parking lot has been destroyed, in the far right corner there is a Magnum, Sniper ammo and a Sniper Rifle.

Another good way to get a Sniper Rifle is to take Sergeant Johnson's before the Pelican picks him up. it is not advised to take his weapon at any time before the Hunter battle as he is an accurate shooter and is a valuable sharpshooter in the courtyard fight.

There is another hidden sniper rifle with ammo on Delta Halo. The area just before you have the battle at the first suspended structure, at the double drone flock battle. There are a bunch of dropped weapons canisters and a beam rifle on the platform at the top of the walk way down to the structure. If you jump up to the landing at the head of the platform/ramp area and move to the foliage in the direction of the water, there is a lower platform with a dead marine, sniper rifle and ammo. Very handy if you want to have more sniping ability from the top of the structure (after grenade jumping subsequent to the next cutscene).

Also on delta halo there is a hidden ODST that is ontop of the first building that you see on the level. when you first drop into the level, kill all the enemies and such then go to the top of the building, look for the ramp, stand on it, then grenade jump up to the roof. there will be a dead ODST with a rocket launcher and a sniper rifle. useful for extra ammo.